1. Field of the Invention
This invention relates to a compound semiconductor luminescent device, and more particularly, it relates to a II-VI group compound semiconductor luminescent device capable of emitting ultraviolet light.
2. Description of the Prior Art
Compound semiconductor luminescent devices capable of emitting ultraviolet light of a short wavelength are usually produced with aluminum nitride (AlN) or boron nitride (BN), both of which are nitride-type III-V group semiconductor compounds. (see, e.g., R. F. Rutz, Applied Physics Letters, 28 (1976) 379 and O. Mishima, Science, 238 (1987) 181.)
FIG. 5 shows a conventional semiconductor compound luminescent device with a metal-semiconductor (MS) structure using AlN. This luminescent device has a structure in which a metal electrode 52 made of niobium (Nb) and a metal electrode 53 made of A1 are disposed on an AlN semiconductor 51. In such a luminescent device, because the AlN semiconductor 51 has a resistivity of about 1000 .OMEGA.cm, current of about 0.1 to 20 mA flows when a voltage of several tens to a hundred of volts is applied to this luminescent device, so that light emission over a wide range of about 215 to 500 nm can be obtained.
FIG. 6 shows a conventional pn junction luminescent device using BN. This luminescent device is produced by growing an Si-doped n-type BN semiconductor 61 and a Be-doped p-type semiconductor 62 under a very high pressure at a high temperature, and by providing the n-type BN semiconductor 61 and the p-type BN semiconductor 62 with silver paste electrodes 63 and 64, respectively. When a voltage of several tens of volts or more is applied to such a luminescent device, current of 0.1 to 3 mA flows, so that light emission over a wide range of about 215 to 600 nm can be obtained.
An AlN semiconductor used in the luminescent device shown in FIG. 5 has a problem in that substrate crystals cannot readily be obtained. For this reason, AlN is deposited on a substrate made of, for example, sapphire (Al.sub.2 O.sub.3), to form an AlN crystal film. Thus, the quality of the crystal is very poor.
A BN semiconductor used in the luminescent device shown in FIG. 6 also has a problem that substrate crystals cannot readily be obtained. Therefore, the luminescent device is produced by growing a very small crystal of about 1 mm in diameter, and by forming electrodes on this crystal particle.
As described above, high-quality crystals of the AlN semiconductor and the BN semiconductor cannot be grown by an ordinary epitaxial growth method such as liquid phase epitaxy, vapor phase epitaxy, molecular beam epitaxy (MBE), or metal organic chemical vapor deposition (MOCVD). Therefore, it is very difficult to obtain epitaxially grown single crystals with excellent crystallinity. Accordingly, luminescent devices in which these semiconductor materials are used requires the application of a high voltage of several tens of volts, and the wavelength of light emitted is over a wide range. Furthermore, these luminescent devices have a disadvantage in that the brightness of the emitted light is low. For these reasons, ultraviolet light emitting devices with luminescent characteristics excellent in wavelength selectivity and luminous efficiency cannot be obtained.
In contrast to these nitride-type III-V group semiconductor compounds, ZnS, ZnS.sub.x Se.sub.1-x, and the like, which are II-VI group semiconductor compounds, have an energy gap of a direct transition type. ZnS has a high energy gap of about 3.7 eV (corresponding to about 340 nm) at room temperature, and ZnS0.75Se0.25 has a high energy gap of about 2.95 eV (corresponding to about 420 nm) at room temperature. Furthermore, in the case of ZnS.sub.x Se.sub.1-x, the wavelength of ultraviolet light can be selected in the range of 420 to 340 nm by changing the mole fraction of S (i.e., x) from 0.75 to 0.95. From these reasons, ZnS and ZnS.sub.x Se.sub.1-x are believed to be a suitable material for an ultraviolet light emitting device.
However, in the case of a II-VI group compound semiconductor such as ZnS, ZnS.sub.x Se.sub.1-x, and ZnSe, GaAs, GaP, Si, and the like are used as an epitaxial growth substrate, so that ZnS, ZnSe, and ZnS.sub.x Se.sub.1-x cannot be grown on any of these substrates as a high-quality epitaxial crystal. Thus, luminescent devices obtained cannot be allowed to emit ultraviolet light with high luminous efficiency under well-controlled conditions.